todo.txt

- Change tuple to a named tuple and remove the INDEX constants.
    - The protocol version could be a class attribute as long as it is
      retained when serialized.

- Consider adding check for `__memokey__` before `repr`ing arguments.
  This would allow for classes to specify their behaviour a little
  better.

- Replace Redis lock with pattern from http://redis.io/commands/set

- Should we have a `validator` which runs with knowledge of previous results in
  order to validate that they are still good?

- Decorators for setting etagger, validator, maxage determinator, etc.:

    @myfunc.set_etagger
    def myetagger(...): pass

    - Would it be better to overload the names, so that the decorator is
      @myfunc.etagger instead of @myfunc.set_etagger, or is that more trouble
      than it is worth?

- Rename `etagger` to `etag` with the following symantics:

    memo.get(key, func, etag='an etag')
    memo.get(key, func, etag=etag_func)

    @memo
    def myfunc(...): pass

    @memo.etag
    def myfunc(...): pass

    myfunc.etag(*a, **kw) -> etag

    - The `etag` attribute on the decorated method will likely need to be
      a custom descriptor.

- `namespace`, `region`, `max_age`, `expiry` and `etag` symantics are such that
  when it is callable it is called to determine the value, otherwise it is taken
  as is. `lock` must always be a lock constructor, and cannot be set to a
  specific lock.